The present invention relates, in general, to electronics, and more particularly, to methods of forming semiconductor amplifiers having a variable propagation delay.
In the past, the semiconductor industry utilized various methods and circuits to implement differential amplifiers including differential amplifiers that had a variable propagation delay. Such variable propagation delay differential amplifiers had various applications including use as a ring oscillator. One example of such a variable propagation delay differential amplifier is disclosed in U.S. Pat. No. 5,063,311 issued to Mavin Swapp on Nov. 5, 1991. Typically, such differential amplifiers varied the voltage swing of the output signal in order to vary the propagation delay through the differential amplifier. Varying the voltage swing resulted in a non-linear propagation delay variation. Typically the propagation delay variation had a non-linearity of greater than about ten per cent (10%), that is, the propagation delay variation deviated from an ideal straight line variation. Also, varying the output voltage swing often produced a large voltage swing that caused deep saturation of the input of subsequent circuit elements that received the output voltage. The saturation resulted in detrimental operation of circuits that utilized the differential amplifier. Another problem with prior variable propagation delay differential amplifiers was RMS cycle-to-cycle jitter or jitter. The non-linearity also increased the jitter of the output signal of the amplifier. Typically, varying the voltage swing produce jitter that was greater than about ten pico-seconds (10 psec.).
Some prior differential amplifiers switched the differential amplifier inputs between one or more data signals having different signal paths. Switching the different signal paths in or out also switched the input signals between input signals having various phase relationships and caused additional jitter in the output signal.
Accordingly, it is desirable to have a differential amplifier that has a variable propagation delay, that does not change the voltage swing of the output signal, that has a more linear output signal, and that has reduced jitter in the output signal.